In many motive power systems there is a driving element, typically the source of power such as an internal combustion engine or electrodynamic machine, which is coupled to a driven element which accomplishes some function in response to the power coupled to it from the driving element. The coupling typically comprises a spacer shaft and two hubs which flexibly connect the spacer shaft to the shaft of the driving and driven elements respectively. In relatively high power applications involving several hundreds or thousands of horse power, it is typical to employ for the coupling hubs spline gears, flexible diaphragms, or other similar elements. Because of the power which is transmitted down the shaft and the nature of the hubs and bearings in the driven and the driving elements, very little misalignment can be tolerated without causing rapid deterioration of the hubs or bearings and inducing their early failure. It is accordingly customary to attempt to attain alignment in such systems to a fraction of a degree or better. While such alignment can be achieved in a static or uncoupled condition, when the driven and the driving elements are at rest, such an alignment technique is susceptible to machining errors and the changes in temperature, pressure, loads and bearings that occur when the equipment is operating, with the shafts rotating and with the entire system at thermal equilibrium. Accordingly, the measure of alignment should be accomplished under operating conditions.
Some systems which have been employed to measure alignment of shafts under operating conditions have conventionally employed some form of sensor stationed off the rotating shafts or couplings to detect cyclic variations at the rate of rotation caused by misalignment. Others measure the misalignment of static elements attached to the housings of the driving and driven elements relying on the assumption that their misalignment accurately reflects shaft misalignment. These generally indirect techniques for sensing alignment require extreme precision and calibration in the use and fabrication of critical parts, such as the element whose cyclic motion is being detected, or the points of alignment measurement of the static elements.